Internally those are "persistent data structures" in Clojure which don't duplicate the data but instead mutate a underlying tree that links both the old state a

Internally those are "persistent data structures" in Clojure which don't duplicate the data but instead mutate a underlying tree that links both the old state and the new state Then as a developer you are looking into a certain piece of that tree depending on what bit of the "mutated value" you are viewing I assume this is to help ensure memory usage doesn't balloon but if you need to do mutation there are non-default features for doing that: https://clojure.org/reference/transients https://clojure.org/reference/transients with all the pros and cons of mutation